Polymer Chemistry - Lesson 9

Copolymerization Part 1

Overview

• Topic: Copolymerization
• Topics to be Discussed:
  • Introduction to copolymerization
  • Copolymer nomenclature
  • Popular copolymers
  • Types of copolymerization
    • Chain Growth Copolymerization
    • Ziegler-Natta Copolymerization
    • Block copolymerization
    • Living polymerization
    • Two-pre polymer method
    • Graft copolymerization
  • Summary: Discussion and Learn

LEGO® Bricks: A Case Study

• Problem: Traditional LEGO® bricks made of cellulose acetate suffered from warpage, affecting quality.
  Example of warpage is given
• Solution Needed: A new polymer resin with specific properties is required.
• Required Properties:
  • Dimensional stability
  • Rigidity
  • Toughness
  • Chemical resistance
  • Glossy surface appearance

Discussion Point

What polymer resin could overcome the warpage issue, and how would it be formed?

Introduction to Copolymerization

• Definition: Copolymerization is a cost-effective polymerization method used to meet demanding market and specific end-use requirements.
• Process: Involves two or more distinct monomers.

Commercial Copolymers

• Ethylene Vinyl Acetate Copolymers (EVA)
  • Chemical structure: $[-(CH<em>2-CH</em>2)<em>m-(CH</em>2-CH(OOCCH<em>3))</em>n]$
• Ethylene Vinyl Alcohol Copolymer (EVOH)
  • Chemical structure: $[-(CH<em>2-CH</em>2)<em>x-(CH</em>2-CH(OH))_y]$
  • Application: Popular barrier layer in multilayer packaging.
    • Product contact inner layer
    • Adhesive layer
    • Barrier layer EVOH
    • Adhesive layer
    • Recycling layer/Masterbatch
    • Outer Layer

Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS)

• Description: An engineering thermoplastic offering excellent impact and crack resistance.
• Applications: Widely used for automotive parts and aerospace components.
• Question: Is it a copolymer?

Copolymerization Details

• Process: Monomers react chemically to form new structures, forming a copolymer.
  Unlike polymer blends, which are a mixture of two or more different polymers.
• Commercial Utilization: Most commercial applications of copolymerization are either unspecified or random due to reaction condition limitations.

Copolymer Nomenclature

• Block Copolymers: The order of monomer names corresponds to their sequence in the molecule.
• Graft Copolymers: The first monomer name cited is that of the main chain.
• Unspecified Copolymers: The sequence arrangement is unknown or undefined.

Copolymer Examples

• Poly(isoprene-ran-methyl methacrylate): A random copolymer with isoprene and methyl methacrylate.
• Poly(styrene-co-butadiene): An unspecified copolymer with styrene and butadiene.
• Poly(styrene-alt-maleic anhydride): An alternating copolymer with styrene and maleic anhydride.
• Polystyrene-graft-poly(ethylene oxide): A graft copolymer with a polystyrene backbone and grafted poly(ethylene oxide) chains.
• Polypropylene-block-poly(vinyl chloride): A block copolymer initiated by polypropylene followed by poly(vinyl chloride) chain extension.

Chemical Structure of Copolymers

• Poly(styrene-co-butadiene): Styrene-butadiene rubber; includes degree of polymerization from butadiene and styrene monomers. Represented as: $[-(CH<em>2-CH=CH-CH</em>2)<em>m-(CH(C</em>6H<em>5)-CH</em>2)_n]$
• Poly(ethylene-ran-vinyl acetate) / Ethylene-Vinyl Acetate (EVA) copolymer: A copolymer composed of two or more different repeating units.

Ethylene Vinyl Acetate (EVA) Copolymer

• Ethylene: Petrochemically derived monomer with good flexibility but relatively high melting point.


• Vinyl Acetate Monomer (VAM) Incorporation: Lowers the melting point, making manufacturing more energy-efficient and improving flexibility.


• Benefits of EVA: Good crack and puncture resistance.


• Example Data:
  

• Application: Popular barrier layer in multilayer packaging, superior to aluminum due to excellent gas barrier properties and chemical resistance.
• Properties: Gas barrier and chemical resistance due to hydrogen bonding.
• Challenges: Sensitivity to moisture due to hydroxyl groups (-OH), making it difficult to process.
• Ethylene: Provides processability and water resistance.
• Result: Combines two monomer units to produce a processable thermoplastic with excellent barrier properties.
• Usage Note: Usually the innermost layer in packaging to avoid water contact.

Types of Copolymerization

• Chain Growth Copolymerization: Employs active centers (free radicals, cations, anions).
• Free Radical Copolymerization: Widely used due to versatility.
• Monomer Reactivity: Varies towards other monomers, affected by temperature, pH, and viscosity.
• Other Factors: Monomer concentration and order of addition significantly affect copolymer composition.

Free Radical Copolymerization: Terminal Model

• Terminal Model: Assumes propagating radical reactivity depends solely on the last monomer unit added to the chain.
• Basis: Defines copolymerization reactivity ratios and predicts copolymer composition.

Four Possible Propagation Steps

Live copolymer chain with monomer 1 as terminal monomer unit.

• Propagation can be complex.
• Self-propagating
• Cross-propagating

k11, k12, k21, k22 are rate constants of the respective propagating steps.

Live copolymer chain with m unit of monomer 1 and n unit of monomer 2 bound in the polymer chain and with the active center located on terminal monomer 1.

Monomer Reactivity Ratios

• Definition: Describe how different monomers behave during copolymerization.
• Information Provided: Indicates if each monomer is more likely to react with itself or the other monomer.
• $r<em>{11}$: Rate constant for the propagation of a growing polymer chain with a terminal $M</em>1$ unit and reacting with $M_1$.
• $r<em>{12}$: Rate constant for the propagation of a growing polymer chain with a terminal $M</em>1$ unit and reacting with $M_2$.
• $r<em>{22}$: Rate constant for the propagation of a growing polymer chain with a terminal $M</em>2$ unit and reacting with $M_2$.
• $r<em>{21}$: Rate constant for the propagation of a growing polymer chain with a terminal $M</em>2$ unit and reacting with $M_1$.
• Factors Affecting Monomer Reactivity Ratios:
  • Steric effect of monomers
  • Resonance effect of monomer
  • Polar effect of the monomers
  • Temperature
  • Solvent used
  • Order of monomer addition

Interpreting Monomer Reactivity Ratios

• What does monomer reactivity ratio tell us?
  • if $r_1$ value is greater than 1?
  • if $r_2$ value is smaller than 1?
  • if $r_2$ value is zero?

Predicting Copolymer Types Based on $r<em>1$ and $r</em>2$

• Ideal Copolymerization ($r<em>1r</em>2 = 1$ or $r<em>1 = r</em>2 = 1$ or approximately 1): Random copolymer is formed as all four propagation reactions are equally possible.
• Alternating Copolymer ($r<em>1r</em>2 = 0$ or $r<em>1 = r</em>2 = 0$ or approximately 0): Monomer 1 tends to react with Monomer 2, vice versa.
• Possible Block Copolymer: If $r<em>1 > 1$ and r2 > 1
• If $r<em>1 >> 1$ while $r</em>2 ≈ 0$

Monomer Reactivity Ratio Examples

Analysis

• Which combinations could possibly form a random copolymer? Why?
• Which combinations could possibly form an alternating copolymer? Why?

Example: EVOH Copolymer Synthesis - Two-Step Process

Adapted from A review. Polymer Reviews, 58(2), 209-246.

1. Ethylene and vinyl acetate monomers are copolymerized into poly(ethylene-ran-vinyl acetate).
2. The methanol attacks the carbonyl carbon and breaks ester bonds, leading to the EVOH formation.

Why poly(ethylene-ran-vinyl alcohol) is a two-step process?